Jt-conjugation

A large part of organic and macromolecular chemistry starts with the chemical functionalization of benzene, and benzene units serve us building blocks for important polymers. Naturally, benzene-based aromatic materials also represent an important subclass of jt-conjugaled architectures. Despite some synthetic difficulties related to the generation of structurally well-defined oligo- and poly(phenyl-... 

Table 1 Spin preference of ground state and the calculated singlet-triplet energy separation of some selected Jt-conjugated diradicals...

Despite the success of these simple rules in the Jt-conjugated diradicals, most of them cannot be directly applied to the localized diradicals within the o-framework. In Sect. 5, we will demonstrate that the orbital phase theory works effectively for the localized 1,3- and 1,4-diradicals as well. 

On the basis of the orbital phase continuity/discontinuity in the involved cyclic orbital interactions, some general rules were drawn for the Jt-conjugated and localized diradicals ... 

It is well known that delocalized stable radicals may have potential for the construction of solid state conducting materials. The phenylalenyl radical 40 has been considered a good candidate with its spin density delocalized over 13 carbons in its jt-conjugated system. Unfortunately, 40 exists in equilibrium with its dimer and it decomposes at modest temperatures. To overcome the dimerization problem, Goto et al. and Koutentis et al. synthesized substituted radicals 41 and 42. 

The effects of changing Jt-conjugation at the 4-position on both the rate of isomerization of the initially formed o-quinones to QMs and the reactivity of the quinoids formed from 4-propylcatechol, 2,3-dihydroxy-5,6,7,8-tetrahydronaphtha-lene (2-THNC), hydroxychavicol, and 4-cinnamylcatechol were studied (Fig. 10.6).9 These catechols were selectively oxidized to the corresponding o-quinones or QMs and trapped with GSH. Microsomal incubations with the parent catechols produced only o-quinone-GSH conjugates. However, if GSH was added after an initial incubation period both o-quinone- and QM-GSH conjugates were observed. The results indicate that the extended Jt-conjugation at the para position enhances the rate... 

Another approach to shift absorption bands for the colored form is the extension of Jt-conjugation outside the spiro skeleton. Procedures of molecular designs for such photochromic compounds are shown in Scheme 20. (1) Position for extension of additional -conjugation in spirothiopyran... 

With the help of similar molecular design, spirothiopyranonaphtho-pyrans 19 with absorption band in the near IR (692-850nm) on UV irradiation have been prepared.52 As predicted, the photocoloration of these spirothiopyrans occurs, but the reverse reaction to colorless form does not occur in solution as extension of Jt-conjugation increases. 

Presumably, isoxazoline (175) is thermodynamically more favorable than overcrowded aziridine (176) due to jt,jt conjugation. Elimination of the bromide anion from intermediate (175) is also hindered due to instability of the carbocation that formed. 

A rapid and versatile covalent assembly technique starting from DEE oligomers has provided the 11.9 nm long hexadecameric poly(triacetylene) rod 20.1481 With its linearly conjugated 16 double and 32 triple bonds spanning in-between the terminal silicon atoms, compound 20 is currently the longest linear, fully Jt-conjugated molecular wire without aromatic repeat units in the backbone. 

A transition from ID- to 2D Jt-conjugated structures has important consequences for the n-electron structure.1131 Furthermore, it is challenging to investigate supramolecular architectures formed from disc-type structures. 

Symmetrical Jt-conjugated systems showing inversion of the standard potentials... 

Julolidine (3) and benzoquinuclidine (4) can be considered as aniline derivatives with parallel and perpendicular electron lone-pairs, respectively. Relative to N,N-dimethylaniline (5), the simplest tertiary aromatic amine, the n orbital of julolidine is destabilized by 0.20 eV, while that of benzoquinuclidine is destabilized by 0.80 eV42. In the latter compound there is no n/jt conjugation while in the former it has a maximum value and, accordingly, the splitting of the first and the third IP is much smaller (0.70 eV) than in the former (2.55 eV) compound. 

Gao et al. (2006) considered the data on an electron double resonance spectra of the cation-radical in conjunction with the results of calculation within the DFT. The authors established that the methyl group at the double bond of the cyclohexene ring is responsible for deprotonation of the P-carotene cation-radical. This route of proton elimination produces the most stable radical leaving the Jt-conjugation system to be intact. Deprotonation at the polyene methyl groups would... 

Tetrapyrroles contain an extended jt-conjugated system which is responsible for their use in a wide range of applications ranging from technical (pigments, catalysts, photoconductors) to medicinal (photodynamic therapy) uses. The electronic absorption spectra are governed by the aromatic 18 tt-electron system and typically consist of two main bands. In phthalocyanines the Q band around 660-680 nm is the most intense one accompanied by a weaker Soret band near 340 nm . In porphyrins the situation is reversed with an... 

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

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